In the fertilizer industry, it is of extreme economic importance to convert gypsum or phosphogypsum into elemental sulfur in order to recycle this product back into the industry. Substantial quantities of gypsum, or calcium sulfate dihydrate, are formed during the production of fertilizer, and if not recycled can pile up quickly, presenting a health or aesthetic hazard to the environment. In the reconversion of phosphogypsum back to sulfur, it would also be beneficial if the catalyst and extractant used in the reaction could be recovered and recycled, as well.
The prior art discloses other attempts to recover sulfur from gypsum. In U.S. Pat. No. 4,520,002 (Willis, Jr.), a method is disclosed by which sulfur values present in gypsum are recovered as SO.sub.2 with the concurrent production of lime. This method is characterized by dehydrating the gypsum, vaporizing the sulfur, then superheating the sulfur in a confined space at temperatures of at least 1270.degree. F. In U.S. Pat. No. 3,607,036 (Foeckling et al.) there is disclosed a method of recovering sulfur from gypsum in a two-stage reduction process which involves fluidizing the components at temperatures in the range of about 600.degree. to 900.degree. C. A further attempt is described in U.S. Pat. No. 4,503,018 (Gardner et al.), in which there is disclosed a method of desulfurizing phosphogypsum which involves mixing it with coal, pelletizing the mixture, then heating to evolve sulfur and/or sulfur dioxide. All of these above methods require very high temperature and none of them disclose recovery of the catalyst used in the removal of the sulfur.
Other methods observed in the prior art involve the recovery of sulfur or sulfur compounds from gas streams using nitrogen or ammonium compounds. In U.S. Pat. No. 3,260,035 (Wheelock et al.) a method of recovering sulfur dioxide from calcium sulfate is disclosed in which a stream of nitrogen is passed over gypsum and heated to temperatures around 2200.degree. F. Additionally, Kohl and Riesenfeld (Gas Purification, 4th Ed., Gulf Publishing Co., Houston, pg. 442 (1985)) describe a method of treating gas streams containing hydrogen sulfide in which sulfur is removed from a catalyst by extraction with ammonium sulfide, which produced an ammonium polysulfide solution. In this latter case, this solution had to be further treated to release elemental sulfur and to recover ammonium sulfide for recycle.
At present, there are no known methods or devices for commercially leaching sulfur from catalysts to produce a sulfur end product. It would therefore be highly desirable to develop a method by which sulfur could be recovered from phosphogypsum in such a manner as to render the catalyst suitable for reuse. It would also be desirable to find an extractant for the sulfur which would be similarly reusable, yet safe, ecomonical, and environmentally acceptable as well.